This has more or less the same specs as the Moto G 3rd gen which is 70£ cheaper and runs stock version of Android rather than touchwiz which will give you better performance.

S3 was up to S6 the only phone I actually considered to be good from Samsung (mostly due to the distinct lack of quality competition at the time), (taking into consideration the price), S6 might get there if it's price drops as S4 and S5 did mostly because of that fast emmc memory in it.

Just search mobiles on this site and you will be introduced to many better deals. Also Amazon are a far better company to buy from if you are set on buying a Samsung.

Should I replace my trusty Galaxy S3 with this, the A3, or wait for the S5 to drop in price?

If you want to stay with Sammy I'd sit tight and wait for the S5 to drop in price a bit more.

The Snapdragon 410 chipset in the J5 and A3 is a backwards step from the Exynos 4412 on the S3 IMO.
The SD 410 only supports single channel memory with max bandwidth of 4.2GB/sec versus the Exynos's dual channel 6.4GB/sec bandwidth, not to mention the drop in clock speed.

The S5 was a flagship model and is a significant upgrade for an S3 user (like myself also) with goodies like fingerprint scanner, IP67 rating, 4k video and keeps the SD slot and replaceable battery - the J5 will never be anything more than lower mid-range at best, the A3 offers even less than the J5.

Why are people voting cold? Let me know if there's a better deal at least!

Motorola Moto G 3rd gen for 99£ at voda phone, voda phone smart ultra 6 at Vodafone 115£, and HTC M8s for 225£ at vodaphone or Moto X play for 233£ JL with Nigel o hara price match.
This has more or less the same specs as the Moto G 3rd gen which is 70£ cheaper and runs stock version of Android rather than touchwiz which will give you better performance.
S3 was up to S6 the only phone I actually considered to be good from Samsung (mostly due to the distinct lack of quality competition at the time), (taking into consideration the price), S6 might get there if it's price drops as S4 and S5 did mostly because of that fast emmc memory in it.
Just search mobiles on this site and you will be introduced to many better deals. Also Amazon are a far better company to buy from if you are set on buying a Samsung.

Thank you for the answer! I'll probably wait until after Christmas now to shop around more.

Should I replace my trusty Galaxy S3 with this, the A3, or wait for the S5 to drop in price?

If you want to stay with Sammy I'd sit tight and wait for the S5 to drop in price a bit more.

The Snapdragon 410 chipset in the J5 and A3 is a backwards step from the Exynos 4412 on the S3 IMO.
The SD 410 only supports single channel memory with max bandwidth of 4.2GB/sec versus the Exynos's dual channel 6.4GB/sec bandwidth, not to mention the drop in clock speed.

The S5 was a flagship model and is a significant upgrade for an S3 user (like myself also) with goodies like fingerprint scanner, IP67 rating, 4k video and keeps the SD slot and replaceable battery - the J5 will never be anything more than lower mid-range at best, the A3 offers even less than the J5.

Thank you for the detailed information. I love my S3 so I want to stick with the brand (unless something amazing appears!).

I have had this phone now for 3 weeks and it is the best phone I have ever had,takes great
photos and works really smoothly. Has very good reviews on Car phone warehouse,so we cannot
all be wrong.

Lots of people can be wrong, it happens all the time. Nazi germany, voting David Cameron back in, X Factor... Samsung phones are very poor value for money, wife has had 2 and never again. As mentioned elsewhere their skinning just saps performance

I have had this phone now for 3 weeks and it is the best phone I have ever had,takes great
photos and works really smoothly. Has very good reviews on Car phone warehouse,so we cannot
all be wrong.

Lots of people can be wrong, it happens all the time. Nazi germany, voting David Cameron back in, X Factor... Samsung phones are very poor value for money, wife has had 2 and never again. As mentioned elsewhere their skinning just saps performance

And the Internet Award for 'Somehow Comparing Nazism With A Completely Different Topic' goes to...

I have had this phone now for 3 weeks and it is the best phone I have ever had,takes great
photos and works really smoothly. Has very good reviews on Car phone warehouse,so we cannot
all be wrong.

Lots of people can be wrong, it happens all the time. Nazi germany, voting David Cameron back in, X Factor... Samsung phones are very poor value for money, wife has had 2 and never again. As mentioned elsewhere their skinning just saps performance

And the Internet Award for 'Somehow Comparing Nazism With A Completely Different Topic' goes to...

In no way comparing actually, just pointing out that popularity is not validation, as is amply demonstrated by any number of examples...

Thank you for the detailed information. I love my S3 so I want to stick with the brand (unless something amazing appears!).
PS I think I reported you by mistake!!! :/

LOL - no worries about reporting me in error :D
The S3 has truly been a towering, bulletproof legend of a handset for me as a modder - sad to see independent developer support starting to dwindle now :(

Samsung just have way to much bloatwear. The Samsung skin is too heavy and takes up too much ram.

Just replace the original software with Cyanogenmod.

Sorry but why should anyone have to root their phone, jeopardise phone security and lose the phone warranty, just to make the phone half decent. My vote goes to phones that are great out of the box.

Actually, I don't see much difference in performance between vanilla Touchwiz and Cyanogenmod. Cyanogenmod has loads of other features and improvements compared to plain Android so it's a matter of preference. Vanilla Android is as insecure as Samsung's edition with Touchwiz or Cyanogenmod, but since you have a root rights on demand in the latter, you need to think before clicking when you are tinkering with your phone or apps - so okay you just need to be a bit more careful.
I agree that bloatware put on the phones by some of the operators (half of it is actually badly written) may affect performance. I say it as a Linux / security engineer and an employee of a huge worldwide telecom.

In short if you like the Samsung hardware but aren't super happy about the software, you have a choice (aftermarket ROMs including Cyanogenmod).

Unfortunately there isn't yet an ideal product on the market and even these with a vanilla system that you call great "out of the box" have poor hardware specs such as subpar screen quality, not very ergonomic, bad speaker, bad mic, bad battery, lack of certain sensors or compass, etc. Or even a very annoying thing like the back button on the wrong side. While you can actually fix software problems, hardware always is what it is. Period.

Sorry but why should anyone have to root their phone, jeopardise phone security and lose the phone warranty, just to make the phone half decent. My vote goes to phones that are great out of the box.

There aren't many that are simply 'great' out of the box.
Root, remove bloat, unroot. No loss of security and no loss of warranty on the hardware (and software on a Sammy if you avoid tripping Knox to 0x1)
TBH it's only when you start unlocking the bootloader that your warranty is potentially at risk. Some manufacturers like Sony and Moto even offer factory unlock tools for the bootloader to end users.

I bought a z1 because of the great deal posted on here a few months back, but after that I'll go back to Motorola. Great phones and no need to muck around with rooting etc. Add es file explorer and you have everything you need. It says a lot that I was disappointed not to really see any benefits over my second generation moto g.

I have had this phone now for 3 weeks and it is the best phone I have ever had,takes great
photos and works really smoothly. Has very good reviews on Car phone warehouse,so we cannot
all be wrong.

Lots of people can be wrong, it happens all the time. Nazi germany, voting David Cameron back in, X Factor... Samsung phones are very poor value for money, wife has had 2 and never again. As mentioned elsewhere their skinning just saps performance

And the Internet Award for 'Somehow Comparing Nazism With A Completely Different Topic' goes to...

In no way comparing actually, just pointing out that popularity is not validation, as is amply demonstrated by any number of examples...

Well when I post a topic about a smartphone I don't expect to see a mention of Nazis...Bit of an extreme example, don't you think!

I have had this phone now for 3 weeks and it is the best phone I have ever had,takes great
photos and works really smoothly. Has very good reviews on Car phone warehouse,so we cannot
all be wrong.

Lots of people can be wrong, it happens all the time. Nazi germany, voting David Cameron back in, X Factor... Samsung phones are very poor value for money, wife has had 2 and never again. As mentioned elsewhere their skinning just saps performance

And the Internet Award for 'Somehow Comparing Nazism With A Completely Different Topic' goes to...

In no way comparing actually, just pointing out that popularity is not validation, as is amply demonstrated by any number of examples...

Well when I post a topic about a smartphone I don't expect to see a mention of Nazis...Bit of an extreme example, don't you think!

No-one expects the Nazis, their chief weapon is the element of... No wait, that's the Spanish inquisition

Hmm, I don't think I'd want to root a phone. The main reasons being that I don't completely understand it and I don't want to void the warranty.
I thought that the S3 runs fine! I agree it does come with a lot of bloatware that I'd rather be able to uninstall, but that's my only gripe with the phone.

My wife's s3 was OK until the last update then slowed to an absolute crawl, regardless of cache wiping etc. After that and a galaxy ace before will never buy Samsung again. She's now on a LG G2 which seems to behave itself apart from major crash the other day.

You've reminded me about the charging issue with the battery, which I think was caused by the update...Samsung wanted me to pay for the programming fix even though it wasn't my fault (the warranty had expired). Perhaps I should look for a different brand if Samsung are going to sabotage older phones to force us to get new ones!